While not intending to be limited by theory, poly-aromatics (hydrocarbons containing aromatic rings with two or more fused rings) are believed to be at least partially responsible for soot emissions from typical diesel engines. Current levels of poly-aromatics in diesel fuels, for example, can range around 3 to 4 weight percent and, in some cases, be as high as about 50 weight percent. While desirable levels of poly-aromatics in diesel fuels have yet to be determined, a reduction in poly-aromatic levels in diesel fuels to less than about 3 weight percent and as low as 1 weight percent or less is one approach for limiting soot emissions from diesel engines.
A distillate hydrotreating unit often is used to produce a diesel boiling range hydrocarbon stream having ultra low sulfur levels of about 10 wppm or less of sulfur. However, a substantial reduction in poly-aromatic content may be difficult to reach and maintain using conventional distillate hydrotreaters and their catalyst/reaction conditions. Such units typically are not configured to operate at conditions necessary to achieve and maintain low levels of poly-aromatics.
To obtain substantial reduction in poly-aromatics, a conventional distillate hydrotreating unit typically must be modified to operate at significantly higher pressures and/or temperatures. Alternatively, high-pressure three-phase hydroprocessing units, commonly known as trickle bed reactors, can be constructed to re-process the diesel from existing distillate hydrotreaters. Constructing such new reactor systems employing higher pressure capabilities would present a considerable expense to the refiner. Suitable high pressure processing units may include trickle-bed reactors operating with either a noble metal catalyst or high pressure trickle bed reactors charged with base metal catalysts. Such units often require costly recycle gas compressors to recycle large quantities of hydrogen gas, typically between about 2,000 and about 10,000 SCF/B, which is necessary in these three-phase reactor systems.
Two-phase hydroprocessing (i.e., a liquid hydrocarbon stream and solid catalyst) also has been proposed in some cases to process certain hydrocarbon streams into other more valuable hydrocarbon streams. For example, the reduction of sulfur in certain hydrocarbonaceous streams may employ a two-phase reactor with pre-saturation of hydrogen as an alternative to three-phase systems. See, e.g., Schmitz, C. et al., “Deep Desulfurization of Diesel Oil: Kinetic Studies and Process-Improvement by the Use of a Two-Phase Reactor with Pre-Saturator,” Chem. Eng. Sci., 59:2821-2829 (2004).
These two-phase systems only use enough hydrogen to saturate the liquid-phase in the reactor. As a result, the reaction systems of Schmitz et al. do not provide for decreasing hydrogen levels due to hydrogen consumption during the reaction process, thus the reaction rate in such systems decreases due to the depletion of the dissolved hydrogen. Such two-phase systems as disclosed in Schmitz et al., therefore, are generally limited in practical application and in maximum conversion rates.
Other uses of liquid-phase reactors to process certain hydrocarbonaceous streams require the use of diluent/solvent streams to aid in the solubility of hydrogen in the oil feed. For example, liquid-phase hydrotreating of a diesel fuel has been proposed, but requires a recycle of hydrotreated diesel as a diluent blended into the oil feed prior to the reactor. In another example, liquid-phase hydrocracking of vacuum gas oil is proposed, but likewise requires the recycle of hydrocracked product into the feed to the liquid-phase hydrocracker as a diluent.
Because hydrotreating and hydrocracking typically require large amounts of hydrogen to effect their conversions, a large hydrogen demand is still required even if these reactions are completed in liquid-phase systems. As a result, to maintain such a liquid-phase hydrotreating or hydrocracking reaction in such systems and still provide the needed levels of hydrogen, such prior liquid-phase systems require the introduction of additional diluents or solvents to dilute the reactive components of the feed relative to the amount of dissolved hydrogen. As a result, in such prior systems, the diluents and solvents provide a larger concentration of dissolved hydrogen relative to the feed to insure adequate hydrogen is dissolved in the liquid to effect the conversion rates in the liquid-phase. Larger, more complex, and more extensive liquid-phase reactors are needed in these systems to achieve the desired conversions.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial petroleum hydrocarbon conversion processes, there is always a demand for new methods and flow schemes that provide more useful products and improved product characteristics. In many cases, even minor variations in process flows or operating conditions can have significant effects on both quality and product selection. There generally is a need to balance economic considerations, such as capital expenditures and operational utility costs, with the desired quality of the produced products.